“This is the first time where, instead of comparing separate clocks over something like 30 centimetres, we’re now looking within a single clock,” says Bothwell. Previous measurements had observed the redshift over larger scales by comparing separate clocks, but the JILA team measured it in a single clock. It is this redshift measurement, calculated to a certainty of 21 decimal places, that was predicted by Einstein’s theory. This means if you were to run the clock for the age of the universe – about 14 billion years – it would only be off by 0.1 second, says team member Jun Ye at JILA. At the top of the clock, a second was measured as 10 -19 of a second longer than it was at the bottom. Then they shone a laser on the stack and measured the scattered light with a high-speed camera.īecause the atoms were arranged vertically, Earth’s gravity caused the frequency of oscillations in each group to shift by a different amount, an effect called gravitational redshift. They used optical light to trap these into a vertical stack 1 millimetre high. Tobias Bothwell at JILA in Boulder, Colorado, and his colleagues separated hundreds of thousands of strontium atoms into “pancake-shaped” blobs of 30 atoms.
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